threat found - restart required

within the last two weeks a pop-up message tells me restart needed to complete removal of threat, trojan cryptolocker.

My question first is, is this found as it tries to enter the system or after it has found it's way onto the system and then found with a scan? 

Second, is their a way to see how this is getting to the computer? as it is an e-mail???

Third, apparently I am not reading the details right as I don't know if this is being quarantined or removed. Do run a scan with Malwarebytes after the restart and show no threats.

Info will be appreciated